According to this document, the X-curve is used to translate what the sound engineer hears in his near-field setup to the expected result in a larger theater room. It is not an equalization or band-pass filtering, it is a calibration curve.
The mixing stages for feature films at Parmount that I've been in are the size of mall theaters. They don't use nearfield monitors. It's a full theatrical sound installation. Maybe some places work differently.